Within medical imaging, there are several different methods used to develop images for medical diagnosis of a patient. These methods include ultrasound (US), magnetic resonance imaging (MRI), computed tomography (CT), single photon emission computed tomography (SPECT), and positron emission tomography (PET).
For PET and SPECT imaging, a patient is injected with a radiopharmaceutical. The radiopharmaceutical associated with PET imaging has a radionuclide that produces gamma particle photon pairs with opposing trajectories from positron annihilation. In SPECT imaging, single photons are produced from a radionuclide with a trajectory from the source of activity in the radionuclide.
Current PET Technology traditionally uses two distinct, separate, and opposing detectors to determine a Line of Response (LOR) of a positron emission event. This could be in the form of a ring (as in Whole Body PET) or as paddles in a high resolution PET system. This is needed in order to detect two distinct gamma particles moving in opposite directions. Gamma particles are created from an event where a positron interacts with an electron and annihilation occurs.
When photons impact scintillation crystals, some gamma particles have energy transferred to visible light. This light is detected by a photomultiplier tube (PMT) or a silicon photo multiplier (SiPM). Electrical signals from the PMT or SiPM are used for event and position detection. These signals are typically in a pulse format that are sent to electrical circuits for amplification and pulse height detection.
With PET, when two gamma particles come into contact of each opposing detector, this is known as a true coincidence event. The timing window for this contact being detected between each detector typically has a range between 0 to 8 nanoseconds. A random event occurs when only one of the gamma particles comes into contact with one of the detectors. The random event cannot provide a LOR since two points were not detected to determine a line. A random coincidence event occurs when two gamma particles from two different annihilation events within the coincidence timing window. This can generate the LOR for imaging, but is incorrect since the LOR was created from two independent events and not a common single coincidence event. Random coincidence events can have a negative impact on the performance of a PET imaging machine.
The annihilation event occurs within the Field of View (FOV) in order for the true coincidence event to occur and determine the line of response for PET imaging. The time between the two gamma particles impacting the detectors for scintillation can be used to discriminate for random events. Random coincident events may be discriminated by other methods since they have the same time occurrences as the coincident event.
When gamma particles generated has a trajectory through material, there are three types of interactions that can occur. They are photoelectric process, Compton scattering process, and pair production process. The combined effects from these three processes are known as attenuation. The gamma photons will either pass through the material, be absorbed by the material or change its trajectory and “scatter”. Based on a beam of photons entering into the material of initial intensity (Io), the intensity attenuation of the gamma photons (It) can be determined:It=Ioe−ux Where x is the thickness of the material and −u is the attenuation coefficient. The attenuation coefficient is dependent on the density of the material, and the photon energy of the gamma particle. For positron-electron annihilation and single photon emissions, the photon energy is typically 511 keV.
With SPECT technology, a single photon is emitted from events of radionuclide activity that is injected into a patient. The photons are detected through the use of a gamma camera where a 2D image is captured. The gamma camera uses collimators for line of sight detection of the emitted gamma photon. The camera is moved with different position and angles so that a 3D image can be generated.
As discussed above, conventional PET systems use two separate and opposing detectors for determining true coincidence of annihilation events. With the drawbacks of conventional systems discussed above, it would be desirable to have a single detector that can be used for three dimensional imaging in medical diagnostics. A multi-detector configuration, such as a ring configuration, is not needed with the use of a single detector or detector arrays. This single detector embodiment provides high resolution stationary scans with the detector in close proximity with the patient's body.